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    <title>Document</title>
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    <script>
      /*
        const r = tree('1',456,2,'a', 'b', 'c', 'css', 'html', 'js')
          tree([4, 5, 6])==456
          tree(['css', 'html', 'js'])='css', 'html', 'js'
          tree( ['a', 'b', 'c', 'css', 'html', 'js'])='a', 'b', 'c', 'css', 'html', 'js'
      */
      const arr = [1, [4, 5, 6], 2, ['a', 'b', 'c', ['css', 'html', 'js']]];
      function tree(arr) {
        let r = '';
        arr.forEach((el) => {
          if (Array.isArray(el) === false) {
            r += `[${el}]`;
          } else {
            r += tree(el);
          }
        });
        return r;
      }
      console.log(tree(arr));
    </script>
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